Estimated Fair Value of Notes Based on Quoted Market Prices (Details) (USD $)
In Millions
Sep. 30, 2011
Dec. 31, 2010
Senior Notes Due 2022 [Member]
  
Fair Value Assets And Liabilities Measured On Recurring Or Nonrecurring Basis Disclosure Items [Line Items]  
Fair value of debt$ 741$ 0
6.625% Senior Subordinated Notes due 2014 [Member]
  
Fair Value Assets And Liabilities Measured On Recurring Or Nonrecurring Basis Disclosure Items [Line Items]  
Fair value of debt327333
6.625% Senior Subordinated Notes due 2016 [Member]
  
Fair Value Assets And Liabilities Measured On Recurring Or Nonrecurring Basis Disclosure Items [Line Items]  
Fair value of debt557568
7.125% Senior Subordinated Notes due 2018 [Member]
  
Fair Value Assets And Liabilities Measured On Recurring Or Nonrecurring Basis Disclosure Items [Line Items]  
Fair value of debt630626
6.875% Senior Subordinated Notes due 2020 [Member]
  
Fair Value Assets And Liabilities Measured On Recurring Or Nonrecurring Basis Disclosure Items [Line Items]  
Fair value of debt$ 735$ 733